This invention relates to a tandem type power servo of the fluid differential pressure actuated type having two power pistons connected in series, and more particularly, to a type thereof, wherein axial length is minimized to provide a compact and economical device, aligning the central shafts.
As shown in U.S. Pat. No. 3,083,698, a tandem type power booster generally comprises a tandem piston, having front and rear power pistons 18, 20, a cylindrical member C connected to a valve body E, a push rod F having one end inserted into the cylindrical member to provide sliding contact with the valve body and the other end projected out from a cylindrical recess of a front shell 10. The front power piston 20 is secured to the cylindrical member and the rear power piston 18 is secured to the valve body. The other end of the push rod is sealingly supported by a sealing member accommodated in the cylindrical recess.
In the conventional apparatus, generally, the cylindrical member and the front power piston are made of synthetic resin and the frontward movement of the tandem piston is stopped by the abutting contact between the rear planar end surface of the cylindrical recess and frontward surface of the front power piston or front planar end of a cylindrical member inwardly extending from the front shell.
However, the thickness of the pistons and the cylindrical member is generally large in light of the mechanical strength due to the materials thereof, and further, restriction of the frontward movement of the tandem piston results from the abutting contact, so that the inwardly extended length of the cylindrical recess causes the overall device to be axially long. Therefore, a compact device is not readily obtainable.